It's difficult, however, to find evidence of gratitude in species that can't say "thank you"... it's difficult but not impossible. By observing behavior, researchers can discover a great deal. Scientists believe that the behavior that reflects gratitude is reciprocity. Reciprocity is a mutual agreement; a kind of "I'll scratch your back and you scratch mine" arrangement that benefits both parties and helps to ensure that both of them are more likely to survive. It seems, then, that reciprocity may have been fundamental in the evolution of gratitude. Even though animals can't verbally express their gratitude, they do so by "returning the favor" or exhibiting reciprocity. There's a good deal of evidence to back this up, especially from the human's closest relatives, the primates. Studies have shown that reciprocity in food sharing is common among monkeys. For example, a group of chimpanzees were given a task that required two chimpanzees to pull in a tray of food. The study was designed so that the chimps were free to come and go as they pleased. One chimp was often sitting there, alone, waiting for a partner to arrive to help with the tray of food. The researchers expected that the chimpanzees might choose to help out their friends. But what happened amazed them. "Instead, the chimpanzees were more likely to help out another chimp in need of a partner if that chimpanzee had also helped them in the past." Reciprocity seemed to be more important than friendship, or skill level at accomplishing the task, when it came to choosing who they helped.
The capuchin monkey last shared an ancestor with humans some 35 million years ago so scientists thoughts it might be useful to see how far back gratitude/reciprocity could be traced in primates. These capuchin monkeys were given a task where they could choose between a prosocial option that rewarded themselves and a partner, or a selfish one that only rewarded themselves. The prosocial reward was selected about 60% of the time. But, when the scientists gave them the opportunity for reciprocity, by alternating roles of chooser and partner, the monkeys became significantly more prosocial, now choosing it nearly 75% of the time! Scientists also learned that capuchin monkeys are highly attuned to whether a situation is reciprocal or not, and can remember if another monkey has helped them, choosing to reciprocate to those monkeys, first. In yet another study, it was found that both the capuchin monkeys and four year-old children engage in what is called upstream reciprocity...paying it forward. Those that received a favor were more likely to donate a favor to someone else...regardless of whether or not that other person/monkey had ever been generous to them. Some researchers argue that gratitude is behind the motivation to pay it forward. It does seem to be the case in young humans, capuchin monkeys, and more than a few frogs I know!
So do monkeys really feel gratitude? "If you take away the ability to ask people why they are helping, experiments of gratitude in humans look very much like experiments of reciprocity in other species." The tendency to return favors, with gratitude being the driving force behind the action, is strong between both man and beast. When the similarities are this great, it becomes hard to argue that animals don't feel gratitude.
Science and research has come a long way since Darwin first proposed that gratitude may be a universally-experienced emotion. There's still more that can be studied, of course, but it does seem that the human propensity for gratitude does have deep roots. I think it'll be fun, and of great interest to many, to see exactly how deep those roots really go. Empathy, too, is another factor in our shared history. When these traits are discovered in other animals, there is only one conclusion that I think can be reasonably drawn. And that is the goodness of humans goes deep, perhaps back to the very beginning of time.